(251g) Kinetics of Pressure-Induced Phase Separation in Polystyrene-Acetone Solutions at High Pressures. Binodal and Spinodal Envelopes

Kinetics of phase separation in polymers deals with non-equilibrium relaxation processes that follow the transfer of a system from a thermodynamically stable to a thermodynamically unstable state. These transformations can be induced by a change in temperature, composition, or pressure. Understanding the kinetics of phase separation of polymer mixtures or polymer solutions is important in polymerization and polymer processing and in property modifications. Information on the time scale of phase separation coupled with the knowledge of polymer properties provide the critical criteria needed to form and lock-in the final microstructure of advanced polymeric materials.

In contrast to many studies that have appeared on kinetics of phase separation in polymer blends, relatively little work has been done on the kinetics of phase separation processes in polymer solutions, and even less that are induced by a pressure change. Compared to temperature or compositional quench, the pressure-induced phase separation gives the distinct advantage in that relatively fast quench rates can be achieved uniformly within a given system.

Using a high- pressure time- and angle-resolved light scattering system suitable to carry out both shallow and deep pressure quenches at pressure up to 70 MPa and temperature up to 413 K, we have in the past investigated the kinetics of pressure-induced phase separation in poly(dimethylsiloxane) + supercritical carbon dioxide, polystyrene + methylcyclohexane and polyethylene + n-pentane. We now report on the kinetics of phase separation in polystyrene (Mw = 129 200; PDI = 1.02) solutions in acetone. Polystyrene solutions are one of most frequently studied solutions and acetone has often been used as a model poor-solvent for polystyrene. These solutions are known to display both UCST and LCST phase behavior and pressure increases the polymer-solvent miscibility by lowering the UCST and increasing the LCST.

In this presentation we will present the results obtained using time- and angle-resolved light scattering. A series of controlled pressure quench experiments with different quench depths were conducted at different polymer concentrations (4.0 %, 5.0 %, 8.2 % and 11.4 % by mass) to determine the binodal and spinodal boundaries and consequently the polymer critical concentration. The results show that the solution with a polymer concentration 11.4 wt % undergoes phase separation by spinodal decomposition mechanism for both the shallow and deep quenches as characterized by a maximum in the angular distribution of the scattered light intensity profiles. Phase separation in solutions at lower polymer concentrations (4.0, 5.0 and 8.2 wt %) proceeds by nucleation and growth mechanism for shallow quenches, but by spinodal decomposition for deeper quenches. These results have been used to map-out the metastable gap and identify the critical polymer concentration where the spinodal and binodal envelops merge.

The time scale of new phase formation and growth as (accessed) from the time evolution of scattered light intensities is observed to be relatively short. The late stage of phase separation is entered within seconds after a pressure quench is applied. For the systems undergoing spinodal decomposition, the characteristic wavenumber corresponding to the scattered light intensity maximum was analyzed by power-law scaling. The domain size is observed to grow from 4 to 10 micrometers within 6 s. The domain growth displays elements of self-similarity.